1. Field of the Invention
The present invention relates to the field of brakes, dampers, actuators, resistance devices and motion control devices. More particularly, the present invention relates to devices employing a magnetic serpentine flux path and a responsive material for controlling torque/force in linear (non-piston-cylinder) configured braking systems.
2. Discussion of the Related Art
Controllable materials (e.g., fluids, powders, etc.), as utilized herein, exhibit a change in their rheological behavior (mainly their apparent viscosity) upon the application of an external magnetic or electric field. In particular, the controllable material exhibits a rheology change, i.e., an increase in viscosity or resistance to shear, upon being exposed to a magnetic field. The greater the magnitude of the magnetic field passing through the field controllable material, the higher the shear stress or torque that can be achieved by the MR configured device. The controllable materials themselves are often non-colloidal (i.e. non-homogeneous) suspensions of polarizable small particles that form chain like structures upon the application of the external field. The particle chains are parallel to the field direction and restrict the fluid flow, requiring a minimum field dependent shear stress (called yield stress) for the flow to be initiated. Such controllable materials, often fluids, are respectively referred to herein in a non-limiting manner as magneto-rheological (MR) fluids, and devices that incorporate such fluids, as utilized herein, may be referred to as magneto-rheological devices, field controllable devices, MR-devices, MR-Dampers, or in particular, MR-Brakes.
During the past few decades, the popularity of magneto-rheological (MR) fluids in the industry has been increasing dramatically. MR-brakes, for example, utilize such fluids to enable a braking torque/force by controlling its viscosity. The fluid is normally similar to low viscosity oil but it becomes a thick medium upon exposure to magnetic flux. MR-brakes (MR-dampers, MR-actuators) are quite popular in many applications including prosthetics, automotive, and vibration stabilization owing to the desirable characteristics, such as high force-to-volume ratio, inherent stability, and simple interface between the mechanical and a coupled electrical system. Force-feedback robotics and (haptic) interfaces (the use of the sense of touch in a user interface design to provide information to an end user) also benefit from the high compactness and high torque capabilities of MR-brakes.
MR-brakes in particular, conventionally employ a coil embedded in the piston. In such a design, the magnetic flux goes through a large cross-sectional area reducing the resulting flux density. The only options in the typical configuration(s) to obtain high magnetic flux density on the controllable fluid are to increase the rod radius, coil windings and current. All of these options lead to bulkier designs.
FIG. 1A shows a three dimensional perspective of such an existing (i.e., conventional) “piston” linear MR-brake (generally referenced by the numeral 10) design similar to an ordinary shock absorber. FIG. 1B illustrates the same “piston” linear MR-brake 10 device but now shown as a sectional view configured in a brake assembly (now generally referenced by the numeral 20). As shown in FIG. 1A, the piston 12, having a length (denoted as L) and a diameter (denoted as DP) is directly coupled to an inner diameter rod 14 having a diameter denoted Dr, that moves along with the piston 12 as known to those skilled in the art in such conventional designs.
To explain the movement aspect, FIG. 1B shows the piston 12, rod 14 arrangement now disposed within a housing 16 of the overall brake assembly 20. In these designs, the inner chamber 22 in a shock absorber operation is filled with a desired MR-fluid (not specifically denoted) instead of a viscous oil and the piston 12 is modified to generate a magnetic field by means of a built-in coil 24, as also shown in FIG. 1A. The wall of inner chamber 22 of the housing 16 is configured as the cylinder 28 of the housing and in operation, the piston 12 disposed within the housing cylinder 28 moves linearly along a direction when the MR-fluid is subjected to magnetic fields 26 (as also denoted as arrows) as provided by desired applied currents directed through coil 24. Importantly, the stroke length is such a device is limited by the inner chamber 22 of the housing 16.
The key point to be taken by FIGS. 1A and 1B is that conventionally, operation of such piston-cylinder configurations, i.e., linear MR-brakes currently available on the market and literature, operate in the “flow mode” and thus are able to produce relatively large forces, but the piston-cylinder arrangement in the brake design amplifies the friction force at the fluid gap to a large pressure difference between the faces of the piston. This leads to a significant off-state force which is uncontrollable and is in addition to that what the actuator (i.e., the housing arrangement discussion for FIG. 1B) applies even when the input current is turned off. Such a force is undesirable in many applications, especially in haptics. For example, such an undesirable off-state force (as coupled to a haptic interface that implements such actuators), keeps applying forces on the user's hand even when the user is not interacting with objects in a virtual simulation environment, hence reducing the realism of the interaction.
Accordingly, there is a need in the field of magneto-rheological (MR) devices for a new non-cylinder-piston linear MR-brake configuration that operates in the shear mode of which can be used for any stroke requirement without size modification in the actuator body. Such a desired design would provide for a wide application area and a greater flexibility based in design, including but not strictly limited to, haptic interface designs. The present invention addresses such a need.